Effects of gas diffusion and electric conduction on the performance of a polymer electrolyte membrane fuel cell (PEMFC) were studied in an effort to optimize the channel configurations of flow field plates. The rib and channel widths of flow field plates were varied from 0.5 to 3 mm. The narrower the rib width, the performance of a cell becomes improved in the range investigated. From the results, gas diffusion seems to be a more important factor than electric conduction for the better cell performance. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.